Optical information recording media (optical discs) are roughly classified, based on the recording/reproduction principles, into the three types: a read-only type, a write-once type, and a rewritable type.
FIG. 1 schematically shows a typical configuration of a read-only optical information recording medium (single-layer optical disc). As shown in FIG. 1, a read-only optical information recording medium has a structure including a reflective film 2 composed of Ag, Al, or Au as a main component and a light-transmitting layer 3 which are laminated in order on a substrate 1 composed of a transparent plastic. The substrate 1 includes, for example, a polycarbonate substrate having a thickness of 1.1 mm and a diameter of 12 cm, on which information is recorded by using combinations of bumps and indentations called “lands” and “pits”, respectively. The light-transmitting layer 3 is formed by, for example, bonding a light-transmitting sheet or applying and curing a light-transmitting resin. Recorded data is reproduced by detecting a phase difference or reflection difference of a laser beam applied to an optical disc.
FIG. 1 shows the single-layer optical disc including a layer each of the reflective film 2 and the light-transmitting layer 3 formed on the substrate on which information is recorded by using combinations of lands and pits (recorded data). However, a dual-layer optical disc including a first information recording surface 11 and a second information recording surface 12 as shown in FIG. 2 is also used. In detail, the dual-layer optical disc shown in FIG. 2 has a configuration in which a first reflective film 2A, a first light-transmitting layer 3A, a second reflective film 2B, and a second light-transmitting layer 3B are sequentially laminated on a substrate 1 on which information is recorded by using bumps and indentations referred to as “lands” and “pits”, respectively (recorded data), the information recorded by combinations of the lands and pits in the first light-transmitting layer 3A being different from that recorded on the substrate 1.
For the reflective film used for the optical disc, Au, Cu, Ag, Al, and an alloy containing two or more of these elements as a main component have been widely used.
Among these, a reflective film composed of Au as a main component has the advantages of excellent chemical stability and little change in recording characteristics with time. However, this reflective film is expensive and has the problem that sufficiently high reflectance cannot be obtained for a blue laser (wavelength 405 nm) used for recording/reproduction in BD. In addition, a reflective film composed of Cu as a main component is inexpensive but has the disadvantages of lowest chemical stability among conventional reflective film materials and low reflectance for a blue laser like in the case of Au, thereby causing a limit to application. On the other hand, a reflective film composed of Ag as a main component shows sufficiently high reflectance in the range of 400 to 800 nm, which is a practical wavelength range, and has high chemical stability, and is thus, at present, widely used for optical discs using blue lasers.
Although Al shows sufficiently high reflectance at a wavelength of 405 nm and is more inexpensive than Ag and Au, an Al-base film has lower chemical stability than an Ag-based or Au-based reflective film. Therefore, in order to secure durability, it is necessary to sufficiently increase the thickness of the reflective film, and, for example, in DVD-ROM, the thickness of the Al-based reflective film is sufficiently increased. However, in BD-ROM (read-only blue-ray disc) using a blue laser, increasing the thickness of the Al-based reflective film as in conventional discs has the problem of decreasing the precision of recorded signals (reproduced signals) (i.e., increasing a jitter value), thereby failing to achieve stable reproduction. There is also the problem that although the reflective film has excellent characteristics (initial reflectance and initial jitter value) immediately after formation, these characteristics deteriorate during long-term storage at high temperature and high humidity. Therefore, the reflective film preferably further has excellent durability.
As a technique using an Al-based alloy for a reflective film for an optical disc, for example, Patent Literature 1 discloses an optical information recording medium using, for a reflective film, an Al alloy containing a least one of Ge, Ti, Ni, Si, Tb, Fe, and Ag as an additive element.
However, Patent Literature 1 relates to a technique on the assumption that it is applied to DVD, and thus, for example, when this technique is applied to the BD-ROM, it is considered to be necessary that stable reproduction is realized by increasing the accuracy of recorded signals.